Storing the Sun's Heat

MIT researchers have revealed the way that a molecule discovered in 1996, called fulvalene diruthenium, can store and release the sun's heat on demand, opening the way to a new approach of obtaining and storing heat and energy.

Storing thermal energy in chemical is a major step ahead in terms of portability and storage, and because ruthenium is a rather rare thus expensive material, scientists are now looking for a similar chemical made out of a more common and cheaper material that could become the basis of a new rechargeable battery that would store heat rather than electricity.

This molecule gets in a high-energy state whenever it absorbs sunlight and it can remain stable continually; when adding a catalyst or extra heat, the fulvalene diruthenium goes back to its initial shape, and in the process it releases the accumulated heat.

The process looks very simple but the researchers actually found out that in the middle of it there is another step, during which the molecule has a semi-stable configuration, between its two possible states.

“It turns out there's an intermediate step that plays a major role,” said Jeffrey Grossman, the Carl Richard Soderberg Associate Professor of Power Engineering in the Department of Materials Science and Engineering, and “that was [rather] unexpected”.

But this intermediate step actually explains why the molecule is so stable, the process is easily reversible and also why another element other than ruthenium has not worked so far.

It is this process that makes it possible to have a “rechargeable heat battery” that can repeatedly gather and release heat from the sun or another source, said Grossman.

He added that theoretically, when the heat of a fuel made out of fulvalene diruthenium is released, it could “get as hot as 200 degrees C, plenty hot enough to heat your home, or even to run an engine to produce electricity.”

It is another approach to solar energy, that “stores the heat in the form of a fuel.”

The process is reversible and very stable, and it also can be used anywhere, on demand; “you could put the fuel in the sun, charge it up, then use the heat, and place the same fuel back in the sun to recharge,” added Grossman.

This discovery is very important, still the problem of the rarity and the cost remains so the next step is to find other materials that have the same behavior.

At this research also contributed Yosuke Kanai of Lawrence Livermore National Laboratory, Varadharajan Srinivasan of MIT's Department of Materials Science and Engineering, and Steven Meier and Peter Vollhardt of the University of California, Berkeley.

The work was reported in a paper published in the journal Angewandte Chemie.

In the video, Jeffrey Grossman explains how this material can be used to store and release energy in the form of heat: